1. Field of the Invention
The present invention relates to improvements in the structure of the valve mechanism of a sliding exhaust brake system which is installed in the exhaust pipe of a large-sized vehicle, such as a truck or bus, and acts to close off the exhaust passage for the exhaust gas flowing through the exhaust pipe when the vehicle is going down steep hills, thereby braking the vehicle.
2. Description of the Prior Art
A known valve mechanism of this kind is disclosed in U.S. Pat. No. 4,205,704. The structure of this valve mechanism is shown in FIG. 6, where a main gate consists of a valve member 23 mounted in a valve working space formed inside the housing of the body of an exhaust brake. The valve member 23 is provided with a single exhaust pressure-adjusting hole 24. An air cylinder system includes a piston rod 26. A relief gate 25 is mounted at the front end of the piston rod 26. A liner 27 makes a frictional contact with the surface of the main gate which is loosely mounted on the underside of the relief gate 25, to form an auxiliary valve member that is fitted to the main valve member at 28. When the exhaust passage is opened and closed, the piston rod 26 is pushed up and down, and the front end 26' of the piston rod and the wall surface 25' of the relief gate 25 collide with the side walls 29' and 29', respectively, of the main gate.
In the conventional structure of the auxiliary valve member, the size of the main gate determined according to a limited space makes it impossible to render the exhaust pressure-adjusting hole large. Therefore, the exhaust pressure cannot be lowered sufficiently before the passage is opened. When the passage is opened and closed, the piston rod collides with the main gate as mentioned previously and so the mating portions wear away or cause play between them. As the passage is opened and closed repeatedly and frequently, the lifetime of the manufactured product is shortened. Hence, the structure cannot be employed for a long period.